The present invention relates to interface circuits for coupling supervisory and information signals appearing on a telephone line with signal utilizing equipment, such as a key telephone system.
When a telephone line is idle (on-hook), i.e., not being used to transmit a telephone call, there is a relatively high open circuit impedance at the telephone set so that very little loop current is drawn from the Central Office battery source. When the line becomes busy (off-hook), a significantly lower terminating impedance is connected across the tip and ring conductors of the line, causing a substantially greater loop current to flow. The increased current indicates that the line is in use, and signals the Central Office of the need for establishing dialing and/or voice signal communication between the telephone set and the Central Office. As used herein, the term Central Office is meant to include other supervisory systems such as PBX and ESS.
To prevent the appearance of a false busy condition at the Central Office, it is important that idle telephone lines provide adequate open loop impedance at the subscriber's station. Yet, there is often a need to permanently or semipermanently couple one or more telephone lines to subscriber's equipment such as a control or receiving circuit that is continuously responsive to supervisory and information signals applied to the line at another location. In order to permanently or semipermanently connect such equipment to the telephone line without significantly lowering the open loop impedance, a condition which might cause a false busy indication, impedance isolation must be provided between the line and the various internal circuits of the connected equipment, and between each of the tip and ring conductors of the line and earth ground.
One way in which this requisite impedance isolation has been previously provided is to connect the telephone line to the signal utilizing equipment through a high input impedance amplifier circuit. The high input impedance, such as may be obtained by an operational amplifier circuit, will provide adequate line to equipment impedance isolation, and still provide an output signal of sufficient strength for proper operation of the signal utilizing equipment. So long as the voltage has a predetermined fixed polarity, proper connection of the tip and ring conductors to the input terminals of the equipment insures that the amplifier circuit will be operated in a forward gain mode.
However, a majority of the telephone systems in current use, do not have a fixed polarity on the tip and ring conductors, but rather, use polarity reversal as one form of supervisory signaling. Also, during installation, it is not always easy to identify the leads and properly hook up the telephone line to the customer's equipment so as to establish the proper phase relationship between the line and the equipment. For these reasons, equipment suppliers almost universally incorporate a full wave diode bridge at the input stage of the subscriber's equipment, so the polarity changes on the line are automatically rectified to apply the telephone signal with the proper phase polarity to the circuitry of the equipment. The proper polarity is thus supplied to the circuitry irrespective of supervisory polarity reversals caused by Central Office, and regardless of inadvertent, misconnection of leads during installation.
The insertion of the full wave diode bridge, however, causes undesirable signal coupling characteristics in at least some applications. For example, where the diode bridge is inserted in front of the coupling amplifier, the input impedance of the amplifier may be of such a high level, e.g., 1 megohm or more, in order to meet the above-mentioned isolation requirements, that the forward current flow through the diode bridge is exceedingly small, and is inadequate to cause the diodes to be biased fully on. With the diodes operating in only a marginally forward biased condition, the signal coupling from the telephone line, through the diode bridge, to the input of the amplifier, tends to be a noisy coupling, introducing unwanted, spurious signal fluctuations at the output of the coupling amplifier.
Furthermore, the existence of the full wave diode bridge between the tip and ring conductors of the telephone line and the input of the operational amplifier, results in a certain amount of impedance unbalance in the termination of the telephone line conductors. Such unbalance in turn causes a differential voltage to develop across the input of the amplifier in response to longitudinal voltages and currents occurring on the telephone line. Ideally, the input of the amplifier should provide a high degree of common mode rejection of such longitudinal voltages and currents; however, the existence of an unbalanced termination at the input to the subscriber's equipment, such as caused by the different conducting states of the individual diodes of the rectifier bridge, allow these longitudinals to appear as spurious differential input voltages to the input amplifier.
While these problems of inadequate forward biasing of the rectifying bridge diodes and the unbalance of the terminating impedance of the line can be tolerated in some applications, other telephone systems require more precision resolution of the line voltages. An example of the latter systems is found in a key telephone system (KTS) of the type disclosed and claimed in U.S. Pat. No. 4,132,860 issued Jan. 2, 1979, to Harry R. Rasmussen for a HOLD CONTROL FOR A KEY TELEPHONE SYSTEM; and in U.S. Pat. No. 4,133,985 issued Jan. 9, 1979. to Harry R. Rasmussen et al. for a KEY TELEPHONE SYSTEM. In the KTS disclosed therein, control circuitry is connected to the telephone line for not only sensing and distinguishing between steady state idle and busy conditions on the line, but also for sensing a special, relatively low amplitude fluctuating hold control signal generated and applied to the telephone line by the KTS for local supervision of the system stations. Because of the nature of the hold control signal and the hold control circuitry that is responsive thereto, the amplitude swing of the hold signal is constrained to a relatively narrow range. Unless the hold control signal is transmitted with a relatively high signal-to-noise ratio from the line through the coupling amplifier of the interface circuit to the control circuit, then less than optimum performance is achieved in some installations.
In addition to the need for reliable sensing and discriminating between busy, idle and hold condition signals, the interface circuit should be capable of reliable coupling of ringing signals in which an AC voltage is superimposed on the direct current supervisory signal. While the superimposed ringing voltage is normally of such a large amplitude, that the interface circuit is capable of accurately sensing and converting large amplitude swings into a satisfactory ring indicating signal, nevertheless, the presence of a superimposed ringing signal does pose certain constraints on the construction and required response characteristics of the interface circuit. Thus, any effort to modify the interface circuit for the purpose of improving the coupling of idle, busy and hold indicating signals from the telephone line to the control unit of the key telephone system must also take into account the intermittent presence of large amplitude superimposed ringing signals.
Accordingly, it is an object of the present invention to provide an improved telephone line to equipment interface circuit characterized by a relatively high input impedance, desirable for isolating supervisory signals on a telephone line from the internal impedance of equipment connected to the line, by the capability of accommodating polarity reversals of the DC supervisory signals on the line, and by a relatively high common mode rejection of longitudinal line voltages and currents.
An additional object of the invention is to provide such an interface circuit capable of sensing and distinguishing between changes of the telephone line voltage that are indicative of idle, busy, holding and ringing conditions, and converting such condition changes into output signals that switch between discrete output levels and that are suitable for further processing by digital control circuitry.
Another object of the invention is to provide an interface circuit having the relatively high input impedance required for line to equipment impedance isolation, and at the same time providing a relatively high signal-to-noise ratio suitable for reliably coupling small amplitude fluctuations in the line voltage indicative of the presence of a hold indicating signal generated by a key telephone system.
Still another object of the invention is to provide such a line to equipment interface circuit that has a symmetrical response to normal and reverse polarity signal conditions on the tip and ring conductors of the telephone line, so as to accommodate polarity reversals of either intentional (supervisory signaling) or unintentional (misconnection) causes.